The present invention relates to microorganisms which are capable of reproducible reduction of sulfur in coal and, more particularly, to a process for the microbial desulfurization of coal.
An increasing awareness of the need to utilize alternative energy sources has prompted renewed interest in obtaining satisfactory methods of cleanly and efficiently burning coal. A substantial disadvantage long felt by the industry is the deleterious effect upon the environment which results from th products of coal combustion. Emission of sulfur oxides into the atmosphere has long been of particular concern.
Many grades of coal and petroleum contain large quantities of sulfur compounds which form corrosive air and water pollutant products during combustion. The presence of sulfur in coal appears in three basic forms; as sulfates, pyrite and organic sulfur. Of the three, sulfates are the least significant, comprising less than 0.5 weight percent of the coal. Pyritic and organic sulfur, however, may each constitute as much as 3.5 weight percent of the coal or from 40 to 60% of the total sulfur content, respectively. Thus, it is apparent that removal of an effective portion of this sulfur content prior to coal combustion would substantially reduce the emission of sulfur oxides into the atmosphere.
Commercial attention has primarily been focused upon the removal of pyritic sulfur which has been relatively easy to facilitate. These processes include numerous physical and mechanical techniques such as heavy media separation, selective agglomeration, floatation, jigging, magnetic separation, leaching and hydrosulfurization. However, the removal of organic sulfur has not met with the same success. By organic sulfur, is meant, that sulfur which is chemically bound within the coal matrix. Organic sulfur is present in four major forms. These are mercaptans or thiols, sulfide, disulfide and aromatic ring sulfur as exemplified by the thiophene system. Since organic sulfur is chemically bound within the coal molecule, effective removal techniques employed must be essentially chemical or biological.
Processes for the biological removal of sulfur from coal are known in the art but until recently have only been successful for the oxidation of pyritic sulfur. For example, such a process is disclosed by U.S. Pat. No. 4,206,288 which employs an iron and sulfur oxidizing microorganism selected from the Thiobacillus ferroxidans group. Microorganisms chosen from this acidophilic group have long been known to have the capability of oxidizing the sulfide and ferrous iron in ores containing sulfide minerals. For example, U.S. Pat. No. 2,829,964 discloses a process for leaching metals such as copper from mixed oxide and sulfide copper ores utilizing an acid solution containing these bacteria.
Oxidation of the sulfide and/or ferrous iron to sulfate and ferric iron can be utilized to release numerous other metals from the mineral lattice. These include iron, nickel, zinc, molybdenum, copper, uranium and aluminum. Other processes which employ Thiobacillus ferroxidans for these known capabilities are disclosed, for example, in U.S. Pat. Nos. 3,218,252, 3,226,899 and 3,305,353. None of the aforementioned references, however, show any effect on the reduction of organic sulfur, although, as indicated, pyritic sulfur has been oxidized and thereby removed from coal with some success.
Methods for the removal of organic sulfur from coal are known but are quite expensive and particularly inefficient, usually employing extreme conditions such as high pressure and high temperature. Typically, the coal product treated by these processes is substantially altered and often may not be utilized for its intended purpose.
Processes for the removal of organic sulfur compounds by microbial treatment have been recently investigated. For example, U.S. Pat. No. 4,562,156 discloses a mutant microorganism Pseudomonas sp. CBI having a registry number ATCC 39381 to be effective in removing organic sulfur compounds from coal. Copending U.S. application Ser. No. 495,657, filed May 18, 1983, discloses a mixed culture of microorganisms comprising one or more of Pseudomonas, Acinetobacter, Azotobacter, and Flavobacteria having a registry number ATCC 39327 which is extremely effective in the removal of organic sulfur from coal.
Removal of organic sulfur compounds by microbial treatment has many advantages. Since high temperature, high pressure or corrosion resistant equipment are not required for the biological process, inexpensive construction materials can be used leading to low capital costs. Moreover, biological treatment is not likely to significantly alter the structure and the composition of the coal or to substantially reduce the BTU value of the coal.